The isotropic hyperfine coupling (hfs) constants of the muonium substituted ethyl radical CH(2)MuCH(2) have been studied at T = 25 K by a Feynman path integral Monte Carlo (PIMC)-density functional (DFT) formalism. The DFT Hamiltonian employed is of the B3LYP-EPRIII type. The hfs constants of the rotationally hindered CH(2)MuCH(2) isomer have been compared with the hfs constants of C2H5 which exhibits free internal rotation. The dissimilar rotational dynamics in both radicals leads to significant differences between some of the isotropic hfs parameters. The PIMC results show the strong coupling between nuclear fluctuations and calculated hfs constants. (C) 2004 Elsevier B.V. All rights reserved.